They want you to have a minimum of 2 inches clearance between the top of the victor fans and anything else, this causes the air to circulate better and which prevents overheating and possible damage to the victor...and at a replacement costing $114.95, I wouldn't want to take any risk in damaging it...

It isn't an official FIRST rule, but its a recommendation from IFI. Unless you can't avoid this, I highly recommend taking that into consideration.